Pilot Jet Type Identification And Rebuild Kit Questions


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Florida, US
Hey guys, I bought the carb rebuild kits from Mike's for my 79 400F. I noticed it came with a 42.5 pilot and a 142.5 main. Why wouldn't it come with a stock size main (137.5)? Is 142.5 just a really common size for "standard" mods? The mains that came out of my carbs are unmarked, so I have no idea what they are. I figured I'm best off going back to stock and starting over (the bike has never run under my ownership, this is a project).

Now, the pilots that came out of my carbs are 45s, and they're the perforated version. The pilots that came in the rebuild kit from Mikes are the solid type.

Since I don't know what the previous owner did jet wise, is there any way to tell what type of pilot jets are supposed to be in there? I'm putting together a jet order from Jets R Us and have read through their identification guide, but that's kind of out the window if the previous owner just swapped jets out willy nilly (i.e. he put in the perforated ones but it was supposed to be solid).

I also ordered a petcock rebuild kit from Mike's, but once I took mine apart I realized that the Mike's kit was missing the gasket for the fuel outlet (4 screw plate) and the gasket at the tank was different than the Mike's kit, though still might work (oval vs oval with two extra bars by the mounting screws). So I'm gathering that even though Mike's lists fitment for parts/kits, it's not perfect.
You have to be really careful with any generic rebuild kits for carbs and petcocks. I definitely wound not use any jets that came with the kit, so good on ya for getting those from jets-r-us. Stock jetting for your year is listed in the factory service manual posted at the top of the Garage.

Also, petcock flange size changed over the years of production to a larger size (don’t recall the exact year though), so you will have to take care with that also.
So any way to know which version of the pilot my carbs are supposed to have? It has a the type with 6 holes but it's not the stock size so don't know if the previous owner put the right jet in.


And yeah regarding the petcock, when I saw the Mike's kit was missing a gasket, I got a kit off eBay for the XS650 that looked better. It had the missing gasket, and also the correct selector and petcock to tank gaskets.


Mike's kit on the left, random ebay kit on the right. The big o-ring in the Mike's kit is meant to be the tank seal. Also the smaller round one for the selector was just a round oring in the Mike's kit. But the one that came off the bike was a tapered square cut o-ring. Anyway, that's all sorted now so I have the Mike's XS400 petcock kit for sale if someone wants it haha.
I am traveling, so don’t have access to all the info at my fingertips, but the Mikuni jet type codes are listed in the specs for each year in the factory service manual. I seem to think it will be the BS30/96 jet, but don’t take my word for it.

Good job on the petcock rebuild. Late model 400s share petcocks and carb bodies with the 650s, so shopping for those opens up more options to get correct fitting aftermarket bits.
I don't see anything for the type, just the size, in both the 360C manual and 400F supplement. But BS30/96 is what came out of my carb so I guess I'll just go with that and hope the PO did things right. Thanks for the help!